The present invention is related to audio processing and particularly, to audio processing with overlapping windows for an analysis-side or synthesis-side of an audio signal processing chain.
Most contemporary frequency-domain audio coders based on overlapping transforms like the MDCT employ some kind of transform size switching to adapt time and frequency resolution to the current signal properties. Different approaches have been developed to handle the switching between the available transform sizes and their corresponding window shapes. Some approaches insert a transition window between frames encoded using different transform lengths, e.g. MPEG-4 (HE-)AAC [1]. The disadvantage of the transition windows is the need for an increased encoder look-ahead, making it unsuitable for low-delay applications. Others employ a fixed low window overlap for all transform sizes to avoid the need for transitions windows, e.g. CELT [2]. However, the low overlap reduces frequency separation, which degrades coding efficiency for tonal signals. An improved instant switching approach employing different transform and overlap lengths for symmetric overlaps is given in [3]. [6] shows an example for instant switching between different transform lengths using low-overlap sine windows.
On the other hand low-delay audio coders often employ asymmetric MDCT windows, as they exhibit a good compromise between delay and frequency separation. On encoder-side a shortened overlap with the subsequent frame is used to reduce the look-ahead delay, while a long overlap with the previous frame is used to improve frequency separation. On decoder-side a mirrored version of the encoder window is used. Asymmetric analysis and synthesis windowing is depicted in FIGS. 8a to 8c. 